The long-term objective of this ongoing project remains the development of appropriate statistical techniques for the analysis of data arising from assays under study in the Environmental Mutagen Test Development Program (EMTDP). Results are also applicable to other large mutagenicity studies, such as the European Collaborative Study and the International Program for the Evaluation of Short-Term Tests for Carcinogenicity. The major focus to date has been on microbial test systems, although preliminary work has now commenced on Drosophila and two mammalian cell systems, CHO and L5178Y. Statistical procedures for the design and analysis of short-term tests proposed or currently employed by other researchers in mutagenicity continue to receive scrutiny, and new and improved procedures continue to be devised.